This invention relates to improvements in electric power assisted steering apparatus and in particular to gearbox assemblies for use in electric power assisted steering systems of the kind which incorporate a worm and wheel gear assembly for transferring torque from an electric motor to a steering shaft or output shaft operatively connected thereto.
It is known to provide a power steering system for a vehicle comprising an electric motor having a stator and a rotor, an input shaft operatively connected to the rotor and adapted to rotate therewith, an output shaft associated with a steering shaft, and a gearbox adapted to transfer torque from the input shaft to the output shaft in response to a measure of the torque in the output shaft as produced by a torque sensor. The motor is typically operated to apply an increasing torque to the output shaft as the measured torque increases, thus applying assistance that helps reduce the effort needed to steer the vehicle.
In a simple arrangement the input shaft carries a worm gear, and the output shaft carries a wheel gear. The teeth of the worm and wheel intermesh to transfer the torque. This system is simple and robust whilst providing relatively high gearing with a low component count. There exists, however, a problem in such gearboxes with noise and vibration due to incorrect meshing between the worm and wheel.
A worm and wheel gearbox, for an Electric Power Steering system, having a fixed centre distance between the axes of the worm and gear wheel is prone to rattle when transmitting zero or low torque if there is excessive clearance between the gear teeth of the worm and those of the gear wheel. Alternatively, it is prone to having high friction if there is forced meshing between the worm and wheel due to interference between the teeth.
A solution to this problem is taught in the applicant's earlier Patent EP 1 087 883 B 1. The document discloses a gearbox assembly that includes a pivoting means at or near to the centre of the wormshaft main bearing, which is at the motor end, in order to allow the wormshaft axis to rotate through small angles about its nominal position in the plane of the gearwheel, typically by less than plus/minus 0.5 degrees. The outer race of the tail bearing is biased towards the gearwheel by a spring with sufficient force to ensure that the worm teeth remain in dual-flank contact with the gear teeth for levels of gearbox output torque of zero to, typically, around 4 Nm. This removes backlash between the worm and gear teeth at low torques and thereby prevents rattle.
The outer race of the tail bearing is usually guided so that it can move only in the plane of the gear wheel. The guidance is provided by a component which is inserted into the gearbox housing and which has a short slot along which the outer race of the bearing can slide, usually by a maximum of around plus/minus 0.5 millimetres. The said guiding component, sometimes referred to as the “Oval Ring” has to be designed so that it itself does not become another source of rattle. It must therefore lightly grip the opposite sides of the tail bearing's outer race without causing enough friction to prevent its ability to slide fairly freely. It must, at the same time, prevent significant deflections of the bearing normal to the gear plane due to the effects of the gear tooth helix angle. These are generally conflicting requirements needing a highly non-linear type of lateral compliance to achieve satisfactory performance.
The oval ring may also incorporate a soft-stop which will prevent an audible impact when, at higher torques, the tail bearing is forced to one end of the slot by the gear tooth separation forces. Whilst that arrangement works well it can result in a small amount of unwanted lateral movement of the bearing together with too much frictional restriction of its movement along the slot.